F. H. Cheng

Anomalous cooling of the massive White Dwarf in U Geminorum following a narrow Dwarf nova outburst

We obtained Hubble Space Telescope Goddard High-Resolution Spectrograph medium-resolution (G160M grating), phase-resolved spectroscopic observations of the prototype dwarf nova U Geminorum during dwarf nova quiescence, 13 days and 61 days following the end of a narrow outburst. The spectral wavelength ranges were centered on three different line regions: N V (1238 A, 1242 A), Si III (1300 A), and He II (1640 A). All of the quiescent spectra at both epochs are dominated by absorption lines and show no emission features. The Si III and He II absorption-line velocities versus orbital phase trace the orbital motion of the white dwarf, but the N V absorption velocities appear to deviate from the white dwarf motion. We confirm our previously reported low white dwarf rotational velocity, V sin i = 100 km s-1. We obtain a white dwarf orbital velocity semiamplitude K1 = 107 km s-1. Using the γ-velocity of Wade, we obtain an Einstein redshift of 80.4 km s-1 and hence a carbon core white dwarf mass of ~1.1 M☉. We report the first subsolar chemical abundances of C and Si for U Gem with C/H = 0.05 times solar, almost certainly a result of C depletion due to thermonuclear processing. This C depletion is discussed within the framework of a weak thermonuclear runaway, contamination of the secondary during the common envelope phase, and mixing of C-depleted white dwarf gas with C-depleted matter deposited during a dwarf nova event. Remarkably, the Teff of the white dwarf 13 days after outburst is only 32,000 K, anomalously cooler than previous early postoutburst measurements. Extensive cooling during an extraordinarily long (210 days) quiescence followed by accretion onto an out-of-equilibrium cooled degenerate could explain the lower Teff.

Hubble Space Telescope Observations of the Dwarf Nova WZ Sagittae: A Very Rapidly Rotating White Dwarf*

We present Hubble Space Telescope Goddard High-Resolution Spectrograph G140L spectra of the white dwarf in WZ Sge, exposed during quiescence. Our best-fitting synthetic spectra yield a rapidly rotating white dwarf with velocity Vrot sin i = 1200+ 300−400 km s-1, white dwarf effective temperature Twd = 14,800 K, the gravity log g = 8.0, and the chemical abundances relative to solar in number (with 3σ error bars) of C, 5.0+ 2.0−2.0; N, 3.0+ 1.0−1.0; and Si, <0.1. We also reexamined a previous Hubble Space Telescope Faint Object Spectrograph G130H spectrum reported by Sion and coworkers and obtained consistent values, but lower C and N abundances. We do not detect a measurable systematic shift in our observed spectra, presumably due to the extremely low mass of the companion.

Evidence of a thermonuclear runaway and proton-capture material on a white dwarf in a dwarf nova

We present Hubble Space Telescope Goddard High-Resolution Spectrograph G160M spectra of the white dwarf in VW Hydri, exposed during quiescence, 1 month after the end of a normal dwarf nova outburst. Our spectra, covering the wavelength interval 1236-1272 A, were obtained at orbital phase 0.06-1.60; they reveal strong photospheric Si II λλ1260, 1265 absorption features and a previously unidentified broad feature centered around 1250 A. This feature is due to a blend of phosphorus lines. From line-shift measurements we determine a gravitational redshift of 58 ± 33 km s-1, yielding a white dwarf mass Mwd = 0.86+ 0.18−0.32 M☉ (this is only the second gravitational redshift determined for a cataclysmic variable white dwarf), white dwarf radius Rwd = 6.5+ 3.1−1.5 × 108 cm, and gravity log g = 8.43+ 0.31−0.54. Our best-fitting synthetic spectra yield white dwarf effective temperature Twd = 22,000 K and a rotational velocity Vrot = 400 km s-1. The chemical abundances, in number relative to solar, are, for C, 0.5; N, 5.0; O, 2.0; Fe, 0.5; Si, 0.1; P, 900; and all other metals, 0.3. The abundance of phosphorus being 900 solar, coupled with the elevated aluminum abundance reported by Sion and coworkers, suggest nucleosynthetic production of these odd-numbered nuclei from proton capture on the even-numbered nuclei during a CNO thermonuclear runaway (TNR) on the white dwarf. It is clear that the white dwarf has undergone a runaway sometime in the past, the first such evidence of a TNR in a dwarf nova. A TNR on a slowly accreting 0.86 M☉ white dwarf should produce a classical nova explosion. If our interpretation is correct, then we have found the first direct spectroscopic link between a dwarf nova and a classical nova by using the white dwarf surface chemical abundance. This is also the first direct evidence of proton capture-processed material in the atmosphere of a white dwarf. Nova explosions on more numerous, lower mass C-O white dwarfs may therefore account for some fraction of the short-lived radionuclide 26Al in the Galaxy. This nuclide is observed from its Galactic gamma-ray line emission and is postulated to have an important role in the heating of small bodies in the solar system.

Hubble Space Telescope Spectroscopy of the Dwarf Nova RX Andromedae. I. The Underlying White Dwarf

We obtained Hubble Space Telescope Goddard High Resolution Spectrograph phase-resolved spectroscopic observations of the dwarf nova RX Andromedae at three times in its outburst cycle: (1) near the end of an extraordinarily deep and long dwarf nova quiescence, 3 months after the last outburst; (2) during the rise to outburst; and (3) near the end of a decline from outburst. The spectral wavelength range covered was 1149-1435 A. All of the spectra are dominated by absorption lines with weak to moderately strong emission wings due to the continued presence of disk material. Uncertainties in line velocities preclude a K1 determination or mass information. Our best-fitting model yielded Twd/1000 = 34.0 ± 0.1 K, log g = 8.0 ± 0.1, and Vrot= 600 km s-1. The Teff value is very similar to the Teff of the white dwarf in U Geminorum, but the rotational velocity appears to be higher than U Gems value. We report approximate subsolar chemical abundances of carbon and silicon for RX And with C being 0.05 times solar and Si being 0.1 times solar while other elements are at essentially their solar values. However, accurate abundances are complicated by line emission, and we cannot exclude the possibility that the abundances of all species are essentially at the solar values. We see no evidence of thermonuclear-processed abundance ratios. If the white dwarf mass is 0.8 M☉, then the corresponding white dwarf cooling age, 4 × 106 years, is a lower limit to the age of this cataclysmic variable (CV). If the peculiar line features seen in the spectrum on the late decline from outburst are inverse P Cygni in nature, then infall velocities of ~ 2000 km s-1 are indicated during the decline from outburst. We compare the surface properties of the RX And white dwarf with the properties of other CV degenerates studied to date with the Hubble Space Telescope, the Hopkins Ultraviolet Telescope, and the International Ultraviolet Explorer.

Hubble Space Telescope Observations of an Accreted Silicon Spot on the White Dwarf in V471 Tauri

We have used the Hubble Space Telescopes (HST) Goddard High-Resolution Spectrograph to detect a photospheric metallic absorption line, Si III λ1206, in the spectrum of the magnetic white dwarf component of the Hyades precataclysmic binary V471 Tauri. The Si III feature is modulated on the soft X-ray/EUV/optical 9.25 minute rotational period of the white dwarf and is strongest at the time of soft X-ray/EUV minimum and optical maximum. A model in which the soft X-ray/EUV magnetic pole is dark because of metallic and helium absorption, and bright in the optical due to flux redistribution, is strongly supported. We derive a Si abundance of 0.10+ 0.03−0.07 times solar in the accretion cap. Assuming equilibrium between mass accretion onto—and diffusion of Si out of—the photosphere, we find the white dwarf to be accreting from its dK companions wind at only 3.8×10−18 M☉ yr-1, some 5 orders of magnitude lower than the Bondi-Hoyle fluid rate. This strongly suggests operation of a magnetic-centrifugal propeller mechanism that rejects most of the material that attempts to accrete. We tentatively detect Zeeman splitting of the Si III line, implying a polar field strength of ~350 kG. V471 Tau is destined, in the distant future, to become a DQ Her-type cataclysmic binary.

Simultaneous Hubble Space Telescope and ASCA Observations of LMC X-4: X-Ray Ionization Effects on a Stellar Wind

We present first results from simultaneous ultraviolet (HST/GHRS) and X-ray (ASCA) observations of the 13.5 s pulsar LMC X-4, taken in 1996 May. The ASCA observations covered 1.12 binary orbits (1.58 days), and the Hubble Space Telescope observations were centered on this for roughly 0.4 orbital phase coverage (0.56 days). The Goddard High Resolution Spectograph data are the highest resolution (both temporal and spectral) ultraviolet spectra ever taken of LMC X-4. With generally accepted parameters for the source, fits to the UV continuum, using a model that incorporates X-ray heating of the companion star and the accretion disk, yield a mass accretion rate =4.0×10−8 M☉ yr-1; the X-ray luminosity implied by this value is consistent with the X-ray flux measured during simultaneous observations (3.2 × 10-10 ergs cm-2 s-1). The model accurately predicts observed B-magnitude and ultraviolet variations over both orbital and long-term periods. The ultraviolet P Cygni lines show dramatic changes with orbital phase, with strong broad absorption near X-ray eclipse and narrow absorption when the X-ray source is in the line of sight. We interpret this as a result of X-ray photoionization of the stellar wind; when the neutron star is in front of the normal star, the wind absorption disappears and mainly the photospheric absorption lines are visible. The X-ray pulse period measured during our observations, 13.5090 ± 0.0002 s, is consistent with steady spin-down over the past 10 years. No pulsations were detected in the ultraviolet observations, with upper limits to the pulsed fraction around N V and C IV of 1.8% and 2.7% in the continuum and 7% and 12% in the absorption troughs, respectively.

HST/STIS spectroscopy of the exposed white dwarf in the short-period dwarf nova EK TrA

We present high resolution Hubble Space Telescope ultraviolet spectroscopy of the dwarf nova EK TrA obtained in deep quiescence. The Space Telescope Imaging Spectrograph data reveal the broad Ly-alpha absorption profile typical of a moderately cool white dwarf, overlayed by numerous broad emission lines of He, C, N, and Si and by a number of narrow absorption lines, mainly of CI and SiII. Assuming a white dwarf mass in the range 0.3-1.4Msun we derive Teff=17500-23400K for the primary in EK TrA; Teff=18800K for a canonical mass of 0.6Msun. From the narrow photospheric absorption lines, we measure the white dwarf rotational velocity, v*sin i=200+-100kms. Even though the strong contamination of the photospheric white dwarf absorption spectrum by the emission lines prevents a detailed quantitative analysis of the chemical abundances of the atmosphere, the available data suggest slightly sub-solar abundances. The high time resolution of the STIS data allows us to associate the observed ultraviolet flickering with the emission lines, possibly originating in a hot optically thin corona above the cold accretion disk.

Temporal Variations of the White Dwarf and Disk in OY Carinae Following the 1992 Superoutburst

Hubble Space Telescope observations of the eclipsing dwarf nova OY Carinae after its 1992 April superoutburst are used to isolate ultraviolet spectra (1150-2500 A at 9.2 A FWHM resolution) of the white dwarf, the accretion disk, and the bright spot. The white dwarf spectra have a Stark-broadened photospheric Lyα absorption feature but are veiled by a forest of absorption features that we attribute to absorption by intervening disk material (a curtain). All the spectral fits required supersonic turbulence in the curtain material with Mach numbers of 6-8. All curtain temperatures were between 10,000 and 11,000 K. There was a curtain temperature increase ~3 months after the superoutburst. We find that the white dwarf temperature changed from 19,700 K just 27 days after the end of the superoutburst to 18,000 K roughly 3 months after the superoutburst; the exponential (e-folding) decay time of the white dwarf temperature was 66 days. We present evidence that the heating of the white dwarf was more extensive during the superoutburst than the normal outburst. The thermal response of the OY Car white dwarf to outburst heating is compared with WZ Sagittae, VW Hydri (the most similar dwarf nova to OY Car), and the cooling timescales of other dwarf novae after superoutburst. The measured cooling timescales of the five systems with superoutbursts appear to be shorter the longer the orbital period (accretion rate). Possible implications are discussed. There is evidence of a disk flux variation, independent of the effect of white dwarf cooling, which suggests a possible contradiction of the disk instability model. To establish this, however, data are required throughout a quiescent cycle.

Far Ultraviolet Rapid HST Photometry of U Gem in Outburst and WZ Sge in Quiescence

Until the advent of HST, the detailed temporal behavior of dwarf novae in the UV spectral range was poorly understood. HST, with its combination of high time and spectral resolutions, has begun to open this field. AE Aqr was recently the subject of high time resolution UV spectroscopy with the Faint Object Spectrograph. The authors found AE Aqr’s coherent 33 second oscillation to have a very large amplitude in the UV (Eracleous et al. 1994). We have undertaken two similar time series studies of U Gem and WZ Sge’s UV photometric behaviors, searching for both coherent periodicities which may arise from the rotation of the accreting white dwarf, and quasi-periodic variations.

Preliminary Analysis of a Hubble FOS Spectrum of VW Hyi in Quiescence: A DAZQ White Dwarf and Accretion Belt/Ring

We present a preliminary analysis of the UV spectrum of the dwarf nova VW Hyi during early quiescence, obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST). The data were obtained approximately one day after the end of a normal outburst.